1. Technical Field
This invention relates generally to a system and method for dispensing aqueous solutions into an elevated temperature environment, and more particularly, to such a system and method for injecting an aqueous urea solution into an exhaust gas stream.
2. Background Art
The selective catalytic reduction (SCR) of NOx using ammonia or urea has been used for many years in industrial processes. For example, U.S. Pat. No. 4,985,218 granted Jan. 15, 1991 to Vincent A. DeVita for a SYSTEM FOR INJECTING A TREATMENT FLUID, SUCH AS A UREA AND WATER SOLUTION, INTO THE EXHAUST DUCT OF A STATIONARY INDUSTRIAL BOILER.
The U.S. Protection Agency (EPA) has set very stringent standards directed toward reducing NOx and other undesirable emissions from vehicular as well as stationary engines, and urea injection upstream of an SCR catalyst has been proposed as one way to reduce NOx emissions from vehicular engines. For example, U.S. Publication No. 205/0013756 A1, published Jan. 20, 2005 for an ENGINE EXHAUST GAS TREATMENT SYSTEM AND EXHAUST GAS TREATMENT PROCESS by Kiyoshi Amou, et al. describes an exhaust gas after treatment system for a diesel engine using urea injection upstream of an SCR selective reduction type NOx catalyst.
However, there are several problems associated with current urea injection systems. For example, in the above-referenced Amou, et al. arrangement, the urea injection valve is directly mounted on a surface that conducts the exhaust gas and is therefore prone to overheating and potential plugging of the valve. Other problems also are inherent in current systems used to inject urea-water mixtures into an elevated temperature exhaust gas. More specifically, the spray of the urea-water mixture does not flow uniformly from multiple holes in the tip of an atomizer positioned in an exhaust pipe, especially at low dosing rates. Furthermore, urea solution supply systems positioned in the exhaust system tend to plug due to partial evaporation of the water in the supply tube between the dosing valve and the atomizer tip. Slugs of urea-water mixture migrate through the supply tube and tend to preferentially spray out of only one or two of the several holes provided in the atomizer tip, a condition particularly troublesome at low liquid flow rates.
The present invention is directed at overcoming the above-described problems associated with urea-water injection. It is desirable to have a urea-water injection system, and a method of carrying out the injection, that avoids the premature mixing of air and the urea solution prior to injection into the exhaust gas stream. It is also desirable to have such a system and method whereby the urea-water solution flow control valve is remotely spaced from the elevated temperature environment of the exhaust system. It is also desirable to have such a system and method for injecting a urea-water solution into an elevated temperature exhaust gas stream that provides a thermally protective continuous flow of air around the urea-water supply tube until the solution is delivered into a small mixing chamber prior to immediate injection into the exhaust gas stream.